It is a well known problem to which there are several proposed solutions.
Systems utilizing balls containing conductive fibres which close circuits when making contact between exposed sensor conductors on the court surface, such as those described in the U.S. Pat. Nos. 3,883,860 and 4,299,384, have the problem that the balls may have to be of different colour, surface texture, friction coefficient, and moment of inertia than that of the now conventionally used tennis ball, and the court will necessarily be of substantially different texture near the lines. Furthermore the circuits may be closed by any electrical conductor such as an aluminium tennis racket, or conductive fibres which may have fallen out of the ball and lie across the sensing conductors on the court.
Systems utilizing electromagnetic interrogation by means of sensing coils buried below the court surface are described in U.S. Pat. Nos. 3,774,194 and 4,664,376.
The former patent describes a radio frequency bridge circuit, one "arm" of which comprises a buried coil, which is used to sense a tennis ball containing a resonant electrical circuit tuned to the radio frequency transmitted by the coil.
The latter patent describes a buried coil which is part of the resonator of an oscillator whose frequency and quality factor are altered by the close presence of a ball of substantially non-zero magnetic permeability.
There are difficulties with such an arrangement in so far that there is susceptibility to external influences. Furthermore, there is great difficulty to assess whether a ball touches the line in cases where the ball is either just "in" or "out"; rather; the described systems merely sense the close proximity of the ball to the sensing coils. In addition, the ball described in U.S. Pat. No. 3,774,194 has physical properties substantially different from a standard ball, such as moment of inertia which necessarily has distinct Elgen axes.
In order to determine whether the ball is "in" or "out", it is necessary to process the received data by means of pattern recognition algorithms or arithmetic algorithms of the data of the received pulse shape generated by the close passage of the ball. Merely measuring the close presence of a ball as indicated in U.S. Pat. No. 4,664,376 will result in inaccuracies for reasons given later.
It is an object of this invention to at least reduce one or more of the above difficulties.